The invention relates to a variable exhaust gas turbocharger of an internal combustion engine with an exhaust gas turbine and a compressor with at least one adjustable flow control element in the suction area. The exhaust gas turbocharger has a housing including a turbine wheel and a compressor wheel connected to the turbine wheel by a common shaft. An engine exhaust duct is connected to the housing to supply exhaust gas to the turbine wheel for generating a drive torque.
Since the beginning of the development of charged internal combustion engines for use in motor vehicles the improvement of the unsteady behavior, particularly in connection with passenger cars, was considered to be of prime importance. The unsteady behavior of a charged internal combustion engine is determined mainly by the properties of the exhaust gas turbocharger. The development of turbochargers was directed, in addition to seeking variability at the turbine side, mainly to a minimizing the mass of the rotor and, at the same time, an increase of the maximum rotor speed of the turbocharger. This means that, with increasing degrees of charging, which will be necessary in the future in connection with emission controls to an increasing degree, very high speed ranges of the exhaust gas turbocharger must be accommodated during transient operations, for example, in city traffic.
The positive effects of the exhaust gas turbochargers on the efficiency of an engine for use in passenger cars are widely recognized. However attention for overcoming the unsteady behavior has mainly been given to improvements concerning reduced wheel volumes. The efficiency potential of the turbocharger which is very important for good operating characteristics of future low-consumption vehicles can not be adequately obtained with the direction of present developments on the charger units for improving the unsteady behavior and increasing the specific engine power output.
DE 199 55 508 C1 discloses a variable exhaust gas turbocharger for an internal combustion engine which includes an exhaust gas turbine and a compressor for generating compressed charge air. In addition, the internal combustion engine includes an air-driven turbine which is connected for rotation with the compressor and to which combustion air can be supplied by way of an adjustable valve structure. The turbine outlet is in communication with the engine intake duct downstream of the compressor by way of a connecting duct. The internal combustion engine includes a control unit for generating control signals for operating the valve structure.
For admitting the air to the air-driven turbine, an additional duct branches off the intake duct upstream of the compressor. The air supply to the additional duct and to the compressor inlet is adjustable by the valve structure. If in a low load range, the desired charge air pressure drops below a threshold value, a control signal is generated in the control unit by which the valve structure is moved to a position in which the additional duct is opened and the air admission to the compressor inlet is reduced.
It is the object of the present invention to provide a variable exhaust gas turbocharger which has an improved efficiency without turbo hesitation (the so-called turbo-lag).
In contrast to the development trend which assumes the high rotational speed changes of the turbocharger in a turbocharged motor as a given fact, the considerations on which the present invention is based are intended to provide an arrangement in which the turbocharger is subjected to only relatively small speed changes so that the turbocharger operates at a high speed level over the whole engine performance range.